As to the load sensing control technique for controlling the displacement of a hydraulic pump so that a differential pressure between a delivery pressure of the hydraulic pump and a maximum load pressure among a plurality of actuators is maintained at a setting value, there are known a pump displacement control system disclosed in JP, A, 5-99126 and a control system for a variable displacement hydraulic pump disclosed in GB Patent 1599233.
The pump displacement control system disclosed in JP, A, 5-99126 comprises a servo piston for tilting a swash plate of a variable displacement hydraulic pump, and a tilting control unit for supplying a pump delivery pressure to the servo piston in accordance with a differential pressure .DELTA.PLS between a delivery pressure Ps of the hydraulic pump and a load pressure PLS of an actuator driven by the hydraulic pump so as to maintain the differential pressure .DELTA.PLS at a setting value .DELTA.PLSref, thereby controlling the pump displacement. The disclosed pump displacement control system further comprises a fixed displacement hydraulic pump driven by an engine along with the variable displacement hydraulic pump, a throttle disposed in a delivery line of the fixed displacement hydraulic pump, and setting modifying means for modifying the setting value .DELTA.PLSref of the tilting control unit in accordance with a differential pressure .DELTA.Pp across the throttle. The setting value .DELTA.PLSref of the tilting control unit is modified by detecting an engine rotational speed based on change in the differential pressure across the throttle disposed in the delivery line of the fixed displacement hydraulic pump.
The control system disclosed in GB Patent 1599233 also has a similar construction. More specifically, a throttle is provided in a delivery line of a fixed pump and a differential pressure .DELTA.Pp across the throttle is introduced to control pressure chambers at opposite ends of a setting adjust valve. When the rotational speed of a prime mover is sufficiently high and the differential pressure .DELTA.Pp is larger than the pressure set by a spring, a valve apparatus 21 establishes communication with the II side and a target load-sensing differential pressure .DELTA.PLSref of a tilting control valve involved in load sensing control is set to a relatively high value. When the prime mover comes into an overload condition and its rotational speed lowers upon change in loads of actuators connected respectively to a plurality of flow control valves, a delivery rate of the fixed pump connected to the prime mover is reduced. If the setting value of the spring becomes higher than the differential pressure .DELTA.Pp across the throttle upon reduction in the pump delivery rate, the setting adjust valve is shifted to establish communication with the I side and the target load-sensing differential pressure .DELTA.PLSref of the tilting control valve involved in load sensing control is set to a relatively low value, thereby relieving a load imposed on the prime mover.